Exploring tunneling ESEEM beyond methyl groups in nitroxides at low temperatures.

Tunneling of methyl rotors coupled to an electron spin causes magnetic field independent electron spin echo envelope modulation (ESEEM) at low temperatures. For nitroxides containing alkyl substituents, we observe this effect as a contribution at the beginning of the Hahn echo decay signal occurring on a faster time scale than the matrix-induced decoherence. The tunneling ESEEM contribution includes information on the local environment of the methyl rotors, which manifests as a distribution of rotation barriers () when measuring the paramagnetic species in a glassy matrix.

Toward a Nanoencapsulated EPR Imaging Agent for Clinical Use.

Purpose: Progress toward developing a novel radiocontrast agent for determining pO in tumors in a clinical setting is described. The imaging agent is designed for use with electron paramagnetic resonance imaging (EPRI), in which the collision of a paramagnetic probe molecule with molecular oxygen causes a spectroscopic change which can be calibrated to give the real oxygen concentration in the tumor tissue.

Procedures: The imaging agent is based on a nanoscaffold of aluminum hydroxide (boehmite) with sizes from 100 to 200 nm, paramagnetic probe molecule, and encapsulation with a gas permeable, thin (10-20 nm) polymer layer to separate the imaging agent and body environment while still allowing O to interact with the paramagnetic probe.

Imaging Reactive Oxygen Radicals in Excised Mouse Lung Trapped by Reaction with Hydroxylamine Probes Using 1 GHz Rapid Scan Electron Paramagnetic Resonance.

Purpose: Oxidative stress is proposed to be critical in acute lung disease, but methods to monitor radicals in lungs are lacking. Our goal is to develop low-frequency electron paramagnetic resonance (EPR) methods to monitor radicals that contribute to the disease.

Procedures: Free radicals generated in a lipopolysaccharide-induced mouse model of acute respiratory distress syndrome reacted with cyclic hydroxylamines CPH (1-hydroxy-3-carboxy-2,2,5,5-tetramethylpyrrolidine hydrochloride) and DCP-AM-H (4-acetoxymethoxycarbonyl-1-hydroxy-2,2,5,5-tetramethylpyrrolidine-3-carboxylic acid), which were converted into the corresponding nitroxide radicals, CP• and DCP•.

Cyclic Disulfide-Bridged Dinitroxide Biradical for Measuring Thiol Redox Status by Electron Paramagnetic Resonance.

Among low-molecular-weight thiols, glutathione (GSH) is the main antioxidant in the cell, and its concentration is an indicator of the redox status. A cyclic disulfide-linked dinitroxide was designed for monitoring GSH by electron-paramagnetic resonance (EPR) spectroscopy. Reaction of the disulfide with GSH and three other thiols was measured at 9.

Electron Paramagnetic Resonance, Electronic Ground State, and Electron Spin Relaxation of Seven Lanthanide Ions Bound to Lanmodulin and the Bioinspired Chelator, 3,4,3-LI(1,2-HOPO).

The electron paramagnetic resonance (EPR) spectra of lanthanide(III) ions besides Gd , bound to small-molecule and protein chelators, are uncharacterized. Here, the EPR properties of 7 lanthanide(III) ions bound to the natural lanthanide-binding protein, lanmodulin (LanM), and the synthetic small-molecule chelator, 3,4,3-LI(1,2-HOPO) ("HOPO"), were systematically investigated. Echo-detected pulsed EPR spectra reveal intense signals from ions for which the normal continuous-wave first-derivative spectra are negligibly different from zero.

Electron spin echo spectra and characterization of electronic ground states for lanthanide ions in acidic water:ethanol glasses.

X-band EPR absorption spectra were obtained by field-swept echo-detection for 10 mM Ce, Nd, Sm, Tb, Er, Tm, and Yb in glassy acidic 1:1 water:ethanol at 4.2 to 6 K. The slowly-varying signals extend over thousands of gauss and are more readily detected by spin echo than by continuous wave EPR.

Direct Measurement and Imaging of Redox Status with Electron Paramagnetic Resonance.

Fundamental to the application of tissue redox status to human health is the quantification and localization of tissue redox abnormalities and oxidative stress and their correlation with the severity and local extent of disease to inform therapy. The centrality of the low-molecular-weight thiol, glutathione, in physiological redox balance has long been appreciated, but direct measurement of tissue thiol status has not been possible hitherto. Recent advances in instrumentation and molecular probes suggest the feasibility of real-time redox assessment in humans.

Synthesis and characterization of a biocompatible C isotopologue of trityl radical OX071 for EPR viscometry.

We describe the synthesis, characterization, and application of an isotopologue of the trityl radical OX071, labeled with C at the central carbon (C). This spin probe features large anisotropy of the hyperfine coupling with the C ( = 1/2), leading to an EPR spectrum highly sensitive to molecular tumbling. The high biocompatibility and lack of interaction with blood albumin allow for systemic delivery and measurement of tissue microviscosity by EPR.

Impact of Chlorine Substitution on Electron Spin Relaxation of a Trityl Radical.

A perchlorotriarylmethyl tricarboxylic acid radical 99% enriched in C at the central carbon (C-PTMTC) was characterized in phosphate buffered saline solution (pH = 7.2) (PBS) at ambient temperature. Samples immobilized in 1:1 PBS:glycerol or in 9:1 trehalose:sucrose were studied as a function of temperature.

Advances in rapid scan EPR spectroscopy.

Progress has been made in hardware for low frequencies, demonstrations of rapid frequency scans, hybrid instrumentation, and improved deconvolution software. The recent availability of the commercial Bruker BioSpin rapid scan accessory for their X-band EMX and Elexsys systems makes this technique available to a wide range of users without the need to construct their own system. Developments at lower frequencies are underway in several labs with the goal of facilitating in vivo and preclinical rapid scan imaging.

Electron paramagnetic resonance characterization and electron spin relaxation of manganate ion in glassy alkaline LiCl solution and doped into CsSO.

Manganate ion, MnO, has important roles in catalysis and potential roles in water treatment. EPR spectra of MnO in a glassy alkaline solution of concentrated LiCl at X-band and Q-band at 80 K exhibit g = 1.9776 ± 0.

Spin-spin interaction and relaxation in two trityl-nitroxide diradicals.

Trityl-nitroxides show substantial promise as polarizing agents in solid state dynamic nuclear polarization. To optimize performance it is important to understand the impact of spin-spin interactions on relaxation times of the diradicals. CW spectra and electron spin relaxation were measured for two trityl-nitroxides that differ in the substituents on the amide linker and have different strengths of the exchange interaction J.

Bis-Spiro-Oxetane and Bis-Spiro-Tetrahydrofuran Pyrroline Nitroxide Radicals: Synthesis and Electron Spin Relaxation Studies.

Synthesis of bis-spiro-oxetane and bis-spiro-tetrahydrofuran pyrroline nitroxide radicals relies on the Mitsunobu reaction-mediated double cyclizations of -Boc protected pyrroline tetraols. Structures of the nitroxide radicals are supported by X-ray crystallography. In a trehalose/sucrose matrix at room temperature, the bis-spiro-oxetane nitroxide radical possesses electron spin coherence time, ≈ 0.

Perchlorinated Triarylmethyl Radical 99% Enriched C at the Central Carbon as EPR Spin Probe Highly Sensitive to Molecular Tumbling.

Soluble stable radicals are used as spin probes and spin labels for in vitro and in vivo electron paramagnetic resonance (EPR) spectroscopy and imaging applications. We report the synthesis and characterization of a perchlorinated triarylmethyl radical enriched 99% at the central carbon, . The anisotropy of the hyperfine splitting with the C ( = 26, = 25, = 199.

Electron Spin Relaxation of Tb and Tm Ions.

Electron spin relaxation times T and T of Tb and Tm in 1:1 water:ethanol and of Tb doped (2%) in crystalline La(oxalate) decahydrate were measured between about 4.2 and 10 K. Both cations are non-Kramers ions and have J = 6 ground states.

Electron paramagnetic resonance of lanthanides.

Many applications of lanthanides exploit their electron spin relaxation properties. Double electron-electron measurements of distances are possible because of the relatively long relaxation times of Gd. Relaxation enhancement measurements of distance are possible because of the much shorter relaxation times of other lanthanides.

Ligand control of low-frequency electron paramagnetic resonance linewidth in Cr(III) complexes.

Understanding how the ligand shell controls low-frequency electron paramagnetic resonance (EPR) spectroscopic properties of metal ions is essential if they are to be used in EPR-based bioimaging schemes. In this work, we probe how specific variations in the ligand structure impact L-band (ca. 1.

Electron spin relaxation of P1 centers in synthetic diamonds with potential as B standards for DNP enhanced NMR.

The microwave magnetic field, B, in the non-resonant structures typically used for DNP-enhanced NMR is relatively small, so calibration via continuous wave (CW) power saturation requires a sample with longer spin lattice relaxation times than the samples used as CW standards in X-band cavities. HPHT diamonds have strong, easily observed EPR signals from P1 centers (nitrogen defects), and are indefinitely stable. This makes HPHT diamonds attractive as secondary standards for calibration of electron B field strength in a variety of experimental arrangements.

Rapid-Scan Electron Paramagnetic Resonance of Highly Resolved Hyperfine Lines in Organic Radicals.

X-band (ca. 9 GHz) fluid solution rapid-scan electron paramagnetic resonance spectra are reported for radicals with multiline spectra and resolution of hyperfine lines as narrow as 30 mG. Highly-resolved spectra of 3-carbamoyl-2,2,5,5-tetramethylpyrrolidin-1-yloxy, diphenylnitroxide, galvinoxyl, and perylene cation radical with excellent signal-to-noise are shown, demonstrating the capabilities of the rapid-scan technique to characterize very small, well-resolved hyperfine couplings.

C isotope enrichment of the central trityl carbon decreases fluid solution electron spin relaxation times.

Electron spin relaxation times for perdeuterated Finland trityl 99% enriched in C at the central carbon (C-dFT) were measured in phosphate buffered saline (pH = 7.2) (PBS) solution at X-band. The anisotropic C hyperfine (A = A = 18 ± 2, A = 162 ± 1 MHz) and g values (2.

Trehalose as an alternative to glycerol as a glassing agent for in vivo DNP MRI.

Purpose: In dynamic nuclear polarization (DNP), the solution needs to form a glass to attain significant levels of polarization in reasonable time periods. Molecules that do not form glasses by themselves are often mixed with glass forming excipients. Although glassing agents are often essential in DNP studies, they have the potential to perturb the metabolic measurements that are being studied.

Supramolecular Approach to Electron Paramagnetic Resonance Distance Measurement of Spin-Labeled Proteins.

We demonstrate a host-guest molecular recognition approach to advance double electron-electron resonance (DEER) distance measurements of spin-labeled proteins. We synthesized an iodoacetamide derivative of 2,6-diazaadamantane nitroxide (DZD) spin label that could be doubly incorporated into T4 Lysozyme (T4L) by site-directed spin labeling with efficiency up to 50% per cysteine. The rigidity of the fused ring structure and absence of mobile methyl groups increase the spin echo dephasing time () at temperatures above 80 K.

Rapid-scan EPR imaging of a phantom comprised of species with different linewidths and relaxation times.

As a demonstration of the application of rapid-scan EPR to imaging at low frequency and magnetic field, a multi-compartment phantom containing six different samples was imaged. The samples were nitroxide radicals, trityl (substituted triarylmethyl) radicals, and the oxygen-sensitive solid lithium phthalocyanine (LiPc), all of which are useful for in vivo imaging. The 2D spectral-spatial image demonstration was performed at 250 MHz, with samples in sealed tubes of various sizes arranged in a 3D-printed plastic holder.